Flexible printed circuits are widely used in consumer and industrial appliances and in appliances for telecommunications. Such circuits comprise a thin, flexible, dielectric layer with conductive layers carried on at least one surface or, more typically, on two opposing surfaces of the dielectric layer. Due to the thin nature of high performance flexible circuits, it is often necessary to adhesively bond such circuits to a support substrate, such as a carrier frame, to ensure that the circuits are not damaged during manufacture and/or use of the appliance. Accordingly, the resulting circuit board assembly comprises three layers with one layer being the flexible circuit, the second layer being the carrier frame, and the third layer being a layer of adhesive between the mating surfaces of the flexible circuit and the frame.
Even in those cases where the flexible circuit is not specifically attached to a carrier frame, it is usually attached to an adjoining substrate by means of an adhesive layer. Unfortunately, this has led to practical limitations in terms of the amount of space occupied by the resulting circuit board assembly since the adhesive layer itself represents a significant part of the thickness of the final product.
Numerous types of adhesives have been used to bond flexible circuits to carrier frames or to other types of substrates. They include pressure sensitive adhesives and single or two component adhesives which may also require cure by heating. Unfortunately, the pressure sensitive adhesives are difficult to handle and are relatively thick. Typically such adhesives have a thickness of at least 2 mils. In addition, there is variation in the thickness of the circuit board assemblies which use such adhesives, due in large part to a variation in the thickness of the pressure sensitive adhesive itself. Accordingly, pressure sensitive adhesives have a relatively limited usefulness in circuit board assemblies.
Heat-curable and air-curable adhesives, which are typically applied between the substrate and flexible circuit by a commercial dispensing machine, are somewhat easier to use but require a considerable amount of time to set. Accordingly, the use of such adhesives is not compatible with high volume production of circuit board assemblies. Moreover, such adhesives squeeze out when pressure is applied during the bonding process, thereby contaminating the outside of the product assembly and making it difficult to precisely control the thickness of the final adhesive layer and the overall thickness of the assembly.
Because of the continued trend towards miniaturization and because of a growing diversity in applications, there are increasingly more stringent requirements in terms of reducing the thickness of circuit board assemblies. Accordingly, a new method for bonding a flexible printed circuit to a substrate which eliminates the need to place an adhesive layer between the mating surfaces of the substrate and the flexible circuit would be highly desirable. A method which employs an adhesive that is easy to use and that requires less than a minute to cure is especially desirable.